Kh. Shaughnessy et Rm. Waymouth, Enantio- and diastereoselective catalytic carboalumination of 1-alkenes and alpha,omega-dienes with cationic zirconocenes: Scope and mechanism, ORGANOMETAL, 17(26), 1998, pp. 5728-5745
Bis(pentamethylcyclopentadienyl) dimethylzirconium (Cp-2*ZrMe2) and bis(1-(
-)-neomenthylindenyl) dimethylzirconium ((NMIn)(2)ZrMe2) upon activation by
tris(pentafluorophenyl)borane are efficient catalysts for the carboalumina
tion of a variety of l-alkenes and nonconjugated dienes. The organoalane pr
oducts can be oxidized to alcohol products, which are isolated in 50-90% yi
eld. (NMIn)(2)ZrMe2 carboaluminates l-alkenes and dienes with moderate (55-
65%) enantioselectivities. In contrast, ethylenebis(tetrahydroindenyl) dime
thylzirconium ((EBTHI)ZrMe2) and related zirconocenes give low activities a
nd selectivities in the carboalumination reaction. Study of the interaction
of zirconocene methyl cations with trimethylaluminum by H-1 NMR shows that
[(EBTHI)ZrMe][MeB(C6F5)(3)] forms a stable bis(mu-methyl) aluminum adduct.
Zirconocene methyl cations derived from Cp-2*ZrMe2 and (NMIn)(2)ZrMe2 reac
t slowly with large excesses of trimethylaluminum to form weakly bound trim
ethylaluminum adducts. The difference in trimethylaluminum binding to the c
ationic zirconocene centers is consistent with the significantly lower acti
vity observed when (EBTHI)ZrMe2 is used as the precatalyst.